Rolling Roads

A rolling road takes its power reading directly from the driven wheels of the car. This means involving gearboxes, drive shafts, differentials and tyres. A lot of people talk about bhp @ the wheels as being the only meaningful number to quote: "It’s what you race with" they will tell you. In a way that is correct, but then the gearbox gets in the way of the true picture. We measure the torque at the wheels but the rpm is measured at the road wheel roller. Put the car in a lower gear and the torque at the wheels increases – but the rpm of the roller is reduced. In theory the resulting bhp should be exactly the same – but it never is. The lower the gear that you run the car in, the higher the bhp at the wheels. This is because we have rolling losses (some call them transmission losses) that increase with increased roller (and hence road wheel) rpm. The biggest single rolling loss is the tyre.

Remember that you have not one, but two contact patches on a rolling road. The tyre has to be compressed in two places and the faster it spins the more often it is compressed. The tyre construction, the diameter and the tyre pressure all have a direct influence on the rolling losses. As an experiment we measured the power at the wheels of a Golf GTi. Then we put another 10 psi into the tyres and checked the power at the wheels again. The power went up by 4bhp! Can you imagine what happens to the rolling losses when the tyre is compressed by several bodies sitting on the back of the car trying to find enough grip to prevent wheel-spin? As long as the bodies stay on the back during the run-down, which measures the rolling loss, you get the right result in the power graph. If the bodies all jump off when the car is knocked out of gear and allowed to run-down, you lose the tyre compression and the losses are less – distorting the resulting graph plot. In order to make any sense of rolling road power figures you must measure the rolling losses and add them to the power at the wheels. When you do that you can run in any gear and get the same result on the power graph – almost.

Several factors prevent you from getting exactly the same result in every gear. First off a lower gear means more torque at the wheels and hence a little more tyre slippage than when you run in a higher gear. The run also takes less time, so the engine accelerates faster and gives you more of a "flash" reading. Our Sun Ram 12 rolling road allows us to alter the acceleration rate so that we can adjust it for different power outputs. The software in our system uses the road speed, measured by the rear roller, to obtain engine rpm in order to scale the power curve. We take an rpm reading at 60mph and the software works out the revs at any given road speed from there. What this doesn’t take into account is tyre growth. As the revs increase the centrifugal force makes the tyre grow – which alters the gearing slightly, putting the rpm out by a tiny amount. When you take all these "fudge factors" into account, it’s a wonder the rolling road is as accurate as it is. But it can be accurate, and more importantly, repeatable.

With careful setting up of the acceleration rate to match the engine power, and accurate setting of the engine rpm, (dashboard tachometers are often out), you can get a meaningful number from a rolling road. I know that when trying to improve the engine in the 'Red Shed' our rolling road is depressingly accurate enough to give the same power curve time after time – despite my best efforts to increase the power output! I always call our final figures "simulated" flywheel figures but they are close enough to engine dynamometers judging by the comparisons we have available.

Ken Snailham at Q.E.D recorded 218bhp on his dyno and the same engine showed 220bhp on our rollers. We’ve had similarly close results to the Lotus Service Centre dyno and J.E. Engineering’s dyno. We also see close to factory-quoted power outputs on most standard cars that we have run in the past.

Mapping with a rolling road or Dynamometer

For mapping, the rolling road’s acceleration mode is all but useless, apart from full throttle runs. You have to be able to switch to fixed (constant) rpm running in order to map and engine. This means operating in closed loop. In closed loop mode the dyno’s absorption unit holds the roller rpm regardless of load. Without a closed-loop rolling road the operator is required to dial in the load in order to hold the rpm. Having done so, the first change to the map that increases engine power also moves the engine to a different speed and load site on the map.

The operator has to re-adjust the load to get back to the target cell. I’m not saying that it can’t be done, but it really needs two people and a lot of time to get a half-decent job done. Letting the electronics do the donkey-work is a lot faster and a lot more accurate. You use the acceleration run for full throttle comparison after the main mapping has taken place. After all, you can’t do full throttle runs until the engine is close to being correctly mapped in the first place. Finally - the arguments for and against dynos and rolling roads... I have worked with both and if you want to develop an engine, the dynamometer is the place to do it. It’s more accurate and a lot more convenient to work just with the engine and nothing else.

But for final mapping I believe the rolling road is the place to do it. The engine runs exactly (wind and ground-effect apart) as it is going to on the track in terms of exhaust.

ROLLING ROAD CHECK LIST

What to make sure of if you are coming for a Rolling Road session.....

Double-check everything you can possibly think of to ensure that your visit to our Rolling Road is productive as well as enjoyable!!